Conventional automotive seats in which the seat back can recline by pivoting relative to the seat cushion are well known to those in the art. The most commonly employed is a type in which the seat back frame has a one-piece construction. This conventional design works well when the seat back is kept in the relatively upright position. However, when the seat back is inclined backward to a position where the occupant's eyes will no longer see straight ahead, then the occupant tends to curve his back away from the seat in order to maintain straight-ahead vision. In this position, the occupant's upper back is no longer supported by the seat back. This causes strain and fatigue on the occupant's back and as a result will reduce the occupant's comfort.
In order to reduce the problems associated with this situation, some seat devices are now designed using a bendable seat back construction instead of a one-piece seat back construction. The bendable seat back has an upper and a lower seat back frame portion which are pivotally connected to each other and have a means of adjusting the angle between the upper seat back frame portion and the lower seat back frame portion. In seat backs of this type, the pivoting is achieved by either a link to a mechanism incorporated with the overall seat reclining mechanism or through an exclusive electric motor mounted adjacent to the pivot point in the seat back frame.
In the motor driven type of bendable seat back, designs have been proposed in which an upper seat back frame is rigidly connected to the upper end portion of mounting brackets which are pivotally connected to a lower seat back frame portion through the means of a nut housing having a threaded shaft, connected to the motor unit, which is mounted on the lower seat back frame portion. The mounting brackets and upper seat back frame portion pivot about the lower seat back frame portion via the nut housing. However, the above-mentioned type of mechanism, which includes the threaded shaft and nut housing, has a drawback in that the mounting bracket pivots in a circular arc whereas the threaded screw and nut combination moves linearly when acted on by the motor. Accordingly, the device needs a mechanism for eliminating this mismatch of movements when pivoting.
This is currently accomplished by slotting the connection between the mounting brackets and the nut housing thereby taking up the positional mismatch by allowing for free play between these two members. However, due to the slotted connection between these two members, the loose fit causes play between the upper seat back frame portion and the lower seat back frame portion. This play can be an annoyance to the occupant who wishes for firm support on his back.